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Head loss and pipe size

Stokehold Member Posts: 43
I am running supply and returns approx. 35 ft. to a radiant floor. I was thinking of using pex-al-pex and was wondering if I should use 1" if my flow rate is only 4 GPM. I am trying to keep over all head loss to a minimum but I am thinking there is a minimum flow through pipe sizes.
I am using a high output LP water heater for my heat source. The tappings on this tank are 3/4". If I can run 1" piping, is there a problem reducing size at the tank?


  • Ironman
    Ironman Member Posts: 7,397
    3/4" pex-al-pex would be sufficient. You want to keep the velocity in hydronic piping between 2 - 4 fps.

    1/2 = 1.5 gpm
    3/4 = 4 gpm
    1" = 8 gpm
    1.25 = 14 gpm
    1.5 = 20 gpm
    2" = 32 gpm

    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
    EBEBRATT-Ed Member Posts: 15,829
    keep in mind that if you oversize pipe (including pex and tubing you risk slowing the velocity (miles/hour) of the water so that any trapped air cannot be removed as it will not be carried back to the air removal device.

    4gpm is about the maximum flow for 3/4" pipe or tubing, so using 1" is fine if you want to keep the head loss low. Either 1" or 3/4" is fine for this application as long as the pex ID is about the same as 3/4 copper.

    The 3/4" water heater tappings will not reduce the flow to any extent